Part 1: Big picture
Drupal 8 will have a fancy new template engine (Twig) and a simplified theme layer (we hope). We'll show you the new template syntax (don't worry, it's easy) and walk through some of the major problems in Drupal 7 that have either already been solved in Drupal 8, or that we are still working hard on improving.
Things we're currently improving include:
Learn about all the changes in the Drupal 8 theme layer, and find out how you can get involved. We want Drupal 8 to be easy for theme developers and people new to Drupal, and also make more sense to the pros.
We still need your help, come learn what we're working on and how to get involved!
Part 2: Nitty gritty
Let's make a Drupal 8 theme together!
We'll make a D8 version of my personal site's theme, including Twig template overrides for page, node, comment, block, field, and more!
The concepts are similar to what you already know about Drupal 7 theme development, but the syntax is a little different, and other minor improvements abound.
Come get your feet wet with Twig, and the new theme layer in Drupal 8.
(completed D8 theme can be reviewed, stolen, or forked from https://github.com/jenlampton/jen)